Fluid Power and Motion Control Blog

There are specific conditions within a hydraulic circuit which could cause a cylinder to slowly creep when its 4-way directional control valve is in its center neutral position. The cause (in the hydraulic circuit) for cylinder drift is the unequal areas on opposite sides of the piston. Oil which leaks across the spool of the 4-way valve, under pressure, acts on unequal areas of the piston to develop a force imbalance which may cause unwanted piston movement unless there happens to be sufficient load against the piston rod to restrain piston drift. Oil leaking internally across the piston seals can also cause drift.

Cylinders which have an oversize or 2:1 ratio piston rod has a greater tendency to drift than those cylinders with smaller (standard) diameter rod. Drifting force is equal to system pressure multiplied times rod area. Therefore, on circuits where drifting may be a problem, the cylinder should have the smallest diameter rod which has sufficient column strength.

Cylinder drift due to hydraulic leaks is not a usual occurrence and is normally the result of oil contamination or component failure. The issues that cause cylinder leaks can occur on any cylinder, and the resulting drift of position can vary based on where the cylinder is installed in the system. The movement as a result of cylinder drift will vary on the load being supported and the position and orientation of the cylinder. Original Equipment Manufacturers (O.E.M.) normally provide specific instructions on the proper inspections and tests needed to allow a trained technician to diagnose the cause of cylinder drift. Proper maintenance and lubrication of your cylinders are essential. When detected, hydraulic issues should be corrected.

The Parker Cylinder Design Engineers understand and have taken steps to help battle this concern

Parker Series RDH industrial grade round line welded hydraulic cylinders provide value through proven reliability, efficient performance and the quality that you expect from every Parker Cylinder Division product line. Each Series RDH hydraulic cylinder incorporates design features that are superior in a 3,000 psi roundline welded cylinder. These features include standard composite wear rings both in the head and on the piston, energized bi-directional piston seal and hard chrome plated piston rods.

The Tri-Lip Rod Seal in combination with the Double-Service Wiperseal™ comprises a sealing system that delivers dry rod performance. The one-piece piston is piloted to the piston rod and sealed with an o-ring to eliminate oil bypass through the joint. Anti-extrusion design o-ring Body End Seals provide leak-free service. Magnetic piston for actuating Parker ALS switches through carbon steel cylinders and EPS or CLS sensors are available for stroke position sensing. Port mounted counterbalance valves add versatility by offering hose break protection, preventing loads from drifting and controlling overrunning loads.

Blocked discharge lines and closed isolation valves are just a couple of ways pressure can build in a positive displacement pump. Unlike centrifugal pumps that will cavitate under these conditions, positive displacement pumps will continue to generate flow and pressure until something gives. Oftentimes it's the pump, resulting in costly component repair or replacement.

(In this example, a closed isolation valve led to a broken gear pump shaft.)

There are many types of positive displacement pumps: gear pump, vane pump, diaphragm pump, piston pump, etc. Regardless of the style of positive displacement pump, a pressure relief valve is needed to 1) protect the equipment, 2) protect from spills and 3) protect the operator.

The RG Group Offers the Right Solutions for Your Applications

The RG Group can provide a wide variety of relief valves for this need. They can work with you to select a pressure relief valve that can fit your desired application.